In the semiconductor industry, there is a desire for higher circuit density in microelectronic devices made using lithographic techniques. Historically, this has been largely accomplished through a combination of wavelength scaling (decreasing the wavelength of the imaging radiation), improvements in the imaging optics (employing lenses with higher numerical apertures), and the use of higher performance photoresists optimized for each new wavelength. The goal is to generate ever-smaller photoresist features with the proper attributes (i.e., vertical shape, etch resistance, etc.), which can then serve as relief images that enable the accurate transfer of a photoimage to the underlying substrate. It is also a requirement that photoresists for 157 nm and EUV lithography function at low exposure doses (have high photospeeds) due to issues of tool throughput and source intensity considerations.
As the semiconductor industry (currently at 248 nm and 193 nm generations) moves to 157 nm and 13.4 nm technology, resist transparency becomes a serious issue. The 157 nm resists currently under development are based on fluorinated polymers with absorbances of 1 to 3 μm−1. Current generation EUV resists (based on phenolic polymers) have values of approximately 2 μm−1. These values are much higher than found in formulated DUV resists (with absorbances of approximately 0.2 to 0.4 micron−1) and pose significant problems for pattern profile control even in the 1000 to 3000 Å thick films required for imaging sub-100 nm features. For example, a change in film transparency from 65% (absorbance of 0.19) to 20% (absorbance of 0.7) has a very large impact on the feature profiles and results in features that have sloping sidewalls and incomplete development to the surface of the substrate. Degraded resist profiles of this type cannot be used in pattern transfer applications.
While major research activities are underway to improve resist transparency at short wavelengths, particularly at 157 nm, it is unlikely that even the best short wavelength resists will be able to achieve the combination of transparency and etch resistance enjoyed by current 193 and 248 nm resists.
There are a number of approaches in the prior art that can potentially be used to address the problem of poor resist profiles. These prior art approaches include multi layer resist systems (also known as thin film imaging resists) employing silicon based polymers or precursors (Willson, C. G. In Introduction to Microlithography 2nd Ed.; ACS Professional Reference book, American Chemical Society, Washington DC.; 1994, Chapter 3; and Miller, R. D., Wallraff, G. M. in Advanced Materials for Optics and Electronics, 1994, 4, 95) can be used to circumvent problems due to highly absorbing resists since image formation occurs in a thin film (in the case of a bilayer resist see U.S. Pat. No. 5,985,524 to Allen, et al.) or in the top surface of the resist. This image is then transferred to the underlying polymer via an anisotropic etch to yield patterns with vertical walls throughout the polymer film(s). As such, this two stage process (imaging followed by O2 anisotropic etch) is fundamentally different from the standard single layer resist process in which the resist relief profile is generated within a single polymer film.
Single layer resists are often used in conjunction with additional polymer films (disposed on top or beneath the imaging layer) to improve image profiles. The primary use of these films is to circumvent problems which are not due to high absorbance but rather due to low resist absorbance. These anti-reflection coatings (ARC's) (Levinson, H., Arnold, W. In Handbook of Microlithography, Micromachining, and Microfabrication, Rai-Choudhury Ed., SPIE Optical Engineering Press: Bellingham, Washington, 1997, 1, Chapter 1) are designed to minimize reflective notching, standing waves and other consequences due to reflectivity at the resist substrate interface. The presence of a bottom ARC (the most prevalent type of reflectivity control system) can unfortunately introduce a different type of profile degradation not linked to resist transparency but rather due to deleterious interaction between the ARC and the chemically amplified photoresist. This interaction (sometimes termed as resist “poisoning”) can result as a thin insoluble resist skin or “foot” at the base of the developed photoresist image (positive tone resist). This effect can be minimized through the incorporation of additives such as acids or photoacid generators. These materials are selected so as to have low diffusivity and thus provide little or no contribution to image formation within the transparent resist film (see U.S. Pat. No. 5,939,236 to Pavelchek et al.).
Alternatively overcoated films containing diffusive basic additives have been disclosed (see Jung et al., application 20010003030) to improve the image profiles in highly absorbing films by neutralizing photoacid at the top of the resist and thus creating a more uniform photoacid concentration throughout the resist film. In this case, the top of the resist film is deliberately “poisoned” requiring that the resist be overexposed (exposed at a higher imaging dose) to achieve vertical profiles. This is an application of the well known consequences of environmental contamination on photoresist profiles (see Hinsberg, W. D., Wallraff, G. M., Allen, R. D. in Kirk-Othmer Encyclopedia of Science and Technology Fourth Edition Supplement 1998).
None of the above mentioned approaches addressees the problem of poor resist profiles in high photospeed semi-transparent resists. It is therefore an object of the present invention to provide an improved process for use in the imaging of semi-transparent resist materials.